I get where you're coming from — certain character types hook a lot of viewers — and there are two different roads to find what you're asking for depending on whether you mean mainstream movies with stepmom characters or adult-themed content that uses the 'stepmom' trope.
For mainstream films, start with the obvious: search services like Netflix, Prime Video, Hulu, HBO Max (now Max), Peacock, Tubi and Pluto. A specific title that actually uses the stepmom theme is 'Stepmom' (1998) with Julia Roberts and Susan Sarandon — emotionally heavy, not erotic — and that one floats in and out of catalogs depending on region. I usually check JustWatch or Reelgood first because they show where a specific title is streaming or available to rent/buy in my country. If you prefer free options, Tubi and Pluto sometimes carry older titles, and library services like Kanopy or Hoopla can surprise you with classic or indie films featuring parental/step-parent dynamics.
If by 'curvy step mom' you were leaning toward adult-oriented content, remember that mainstream platforms won't carry explicit material. In that case, look for legal, paid adult sites that explicitly verify age and operate in your jurisdiction — and do so responsibly. Search terms are blunt there, so be careful and respect local laws. Personally, I usually stick with the mainstream routes for movie nights and use the adult side only when I want something very specific; either way, verifying legitimacy and age restrictions is a habit I recommend. Enjoy whatever tone of movie you choose — I tend to prefer the dramatic takes, but hey, to each their own.